TimeStamper() now uses TZ-aware objects

The default output doesn't change but manual formatting allows for TZ
data now.

Fixes #703

src/structlog/processors.py

@@ -525,8 +525,7 @@ def now() -> datetime.datetime:
     else:
 
         def now() -> datetime.datetime:
-            # A naive local datetime is fine here, because we only format it.
-            return datetime.datetime.now()  # noqa: DTZ005
+            return datetime.datetime.now().astimezone()
 
     if fmt is None:
 
@@ -540,7 +539,9 @@ def stamper_unix(event_dict: EventDict) -> EventDict:
     if fmt.upper() == "ISO":
 
         def stamper_iso_local(event_dict: EventDict) -> EventDict:
-            event_dict[key] = now().isoformat()
+            # We remove the timezone offset for backwards-compatibility. If the
+            # user wants a timezone, they have to set fmt manually.
+            event_dict[key] = now().isoformat().rsplit("+", 1)[0]
             return event_dict
 
         def stamper_iso_utc(event_dict: EventDict) -> EventDict:

tests/processors/test_renderers.py

@@ -396,8 +396,8 @@ def test_inserts_utc_unix_timestamp_by_default(self):
     @freeze_time("1980-03-25 16:00:00")
     def test_local(self):
         """
-        Timestamp in local timezone work.  We can't add a timezone to the
-        string without additional libraries.
+        Timestamp in local timezone work. Due to historic reasons, the default
+        format does not include a timezone.
         """
         ts = TimeStamper(fmt="iso", utc=False)
         d = ts(None, None, {})
@@ -414,6 +414,17 @@ def test_formats(self):
 
         assert "1980" == d["timestamp"]
 
+    @freeze_time("1980-03-25 16:00:00")
+    def test_tz_aware(self):
+        """
+        The timestamp that is used for formatting is timezone-aware.
+        """
+        ts = TimeStamper(fmt="%z")
+        d = ts(None, None, {})
+
+        assert "" == datetime.datetime.now().strftime("%z")  # noqa: DTZ005
+        assert "" != d["timestamp"]
+
     @freeze_time("1980-03-25 16:00:00")
     def test_adds_Z_to_iso(self):
         """